1. Field of the Invention
The present invention relates to a matrix type display unit, and more particularly to a matrix type display unit containing a drive circuit therein.
2. Description of the Related Art
The active matrix type display unit is a display unit in which a pixel is arranged at each intersection of a matrix which is made up of signal lines 1 and scanning lines 2, and a switching element is provided for each pixel in such a manner that pixel information is controlled by turning on/off the respective switching elements, as shown in FIG. 2. Liquid crystal 3 is used as a display medium of the display unit of this type. The switching element may be formed of, in particular, a three-terminal element, that is, a thin-film transistor 4 having a gate, a source and a drain.
Also, in the present specification, a "row" in the matrix is defined by the scanning line 2 (gate line), which is arranged in parallel to a subject row, being connected to a gate electrode of the thin-film transistor 4 of the subject row, and a "column" in the matrix is defined by the signal line 1 (source line), which is arranged in parallel to a subject row, being connected to a source (or drain) electrode of the thin-film transistor 4 of the subject column. Furthermore, a circuit that drives the scanning line 2 is called a "scanning line drive circuit", and a circuit that drives the signal line 1 is called a "signal line drive circuit". Also, the thin-film transistor is called a "TFT".
What is shown in FIG. 3 is a first conventional example of the active matrix type liquid-crystal display unit. The active matrix type liquid-crystal display unit of this example has the TFT using amorphous silicon, and the scanning line drive circuits and the signal line drive circuits which are made up of monocrystal integrated circuits (301, 303), and they are fitted onto the periphery of a glass substrate using tabs as shown in FIG. 3A, or the former are fitted onto the latter through the COG (chip on glass) technique as shown in FIG. 3B.
The liquid-crystal display unit of this type suffers from problems stated below. One problem may arise from the viewpoint of the reliability because the signal lines and the scanning lines of the active matrix are connected to each other through the tabs or bonding wire. For example, in the case where the display unit is of VGA (video graphic array), the number of signal lines is 1920, and the number of scanning lines is 480. The number of those lines shows a tendency to increase year by year as the resolution is improved.
In the case of producing a video camera view finder or a projector using liquid crystal, there is required that the display unit is compacted in a lump. The liquid-crystal display unit using the tabs as shown in FIG. 3A is disadvantageous from the viewpoint of a space.
There has been developed the active matrix type liquid-crystal display unit that solves those problems in which TFT is made of polysilicon. One example of this display unit is shown in FIGS. 4A and 4B. As shown in FIG. 4A, a signal line drive circuit 401 and a scanning line drive circuit 402 are formed on a glass substrate 400 together with pixel TFTs of an active matrix 403, using polysilicon TFTs. The formation of the polysilicon TFT is conducted by a high-temperature polysilicon process in which an element is formed on a quartz substrate through a process at 1000.degree. C. or higher, or a low-temperature polysilicon process in which an element is formed on a glass substrate through a process at 600.degree. C. or lower.
The polysilicon TFT can increase its mobility to 30 cm.sup.2 /Vsec or more whereas the amorphous TFT is about 0.5 cm.sup.2 /Vsec in mobility. Thus, polysilicon TFT can be operated by a signal of about severals MHz.
The drive circuit that drives the active matrix type liquid-crystal display unit is of the digital type and the analog type. The drive circuit using polysilicon is generally of the analog type. It should be noted that because the number of elements in the circuit of the digital type is remarkably more than that of the analog type, the drive circuit using polysilicon is generally of the analog type. Also, the circuit structure of the scanning line drive circuit and the signal line drive circuit generally uses a shift register 405 in which N- delay type flip flop circuits 404 are connected in series (refer to FIG. 4B).
The above-described conventional liquid-crystal display unit suffers from problems stated below. In the TFT using polysilicon, the control of a threshold value is generally difficult in comparison with a monocrystal transistor, and what is naturally to be of the enhancement type becomes of the depletion type so that a current may flow into a drain even though a voltage between a gate and a source is 0. This is because polysilicon is nonuniform in crystallinity more than monocrystal, a thermal oxide film cannot be used for a gate oxide film in the case of the low-temperature polysilicon, impurity contamination is caused, and so on.
For example, assuming that the TFT characteristic which is to be naturally exhibited by FIG. 5A becomes the characteristic shown in FIG. 5B with a shift of the threshold value, in an initial stage of an invertor circuit 600 shown in FIG. 6, no current flows when an input signal is in a high-state, but a current is caused to flow from a power supply to GND when the input signal is in a low-state. Further, current flows in the next stage in a high condition. Also, in the case where the drive circuit for the liquid-crystal display unit is installed in a substrate of a TFT, its stage number becomes 1120 in total at both of a signal side and a scanning side when the display unit is of the VGA type. As a result, even though a small current flows into each of the TFTs; the total value of the current becomes large. This causes a serious problem from the viewpoint of reducing a power consumption of the display unit.
On the other hand, if the threshold value becomes too large, an on-state current of the TFT is decreased, resulting in such a problem that the operating frequency of the drive circuit is lowered. The operating frequency of the drive circuit is determined by the magnitude of the on-state current when a load capacity and a supply voltage are kept constant because the load capacity is driven by the on-state current of the TFT. Hence, the too large threshold value leads to a lowered operating frequency.